The present invention falls generally within the coating arts and relates, more particularly, to components having high temperature oxidation resistant coatings thereon which provide protection in severe environments such as those associated with advanced gas turbine engines.
Coatings of the MCrAlY-type are now well known in the art, as evidenced by the U.S. Patents to Evans et al. U.S. Pat. No. 3,676,085; Goward et al. U.S. Pat. No. 3,754,903; and Talboom, Jr. et al. U.S. Pat. No. 3,542,530; all of which share a common assignee with the present invention. Typically, the MCrAlY coatings are characterized by high chromium and aluminum contents and contain yttrium in a basis metal comprising one or more of the elements selected from the group consisting of cobalt, nickel and iron. They are usually characterized as overlay coatings denoting the fact that they are deposited as the MCrAlY alloy on the surface to be protected and, thus, act substantially independent of the substrate in the performance of their protective function.
Aluminide coatings and processes for producing such coatings are also known and have been used for a number of years as the principal coating technique for gas turbine engine elements. In the U.S. Patent to Joseph U.S. Pat. No. 3,102,044, which also shares a common assignee with the present invention, aluminum rich slurry applied to a superalloy surface is reacted therewith to form a protective aluminide or aluminides. U.S. Pat. No. 3,257,230 describes another aluminizing technique, i.e. the formation of a protective aluminide on alloy surfaces by a pack cementation process.
Prior to the introduction of the MCrAlY-type coatings the superalloys were, as previously mentioned, typically protected through the formation of an aluminide directly on and by reaction with the superalloy surface by exposure of that surface at high temperature to aluminum or aluminum containing vapors. The principal aluminide formed was usually that of the basis metal of the substrate, i.e. nickel, cobalt or iron. However in addition to the principal aluminide the coating layer often included amounts of other ingredients present in the substrate alloy and, in most instances, the total coating comprised a composition which while acceptable nevertheless represented a compromise in terms of composition and something less than would be desired if the coating were to be optimized.
As engine environments and other demands on the coated aluminum increased in severity, the widely used aluminide found less acceptability in some circumstances and it became advisable to pursue further coating improvements. Coatings, of course, play a major role in engine design acceptability. The MCrAlY-type coatings were the result of such coating improvement studies and permitted the engine designer greater flexibility in his constructions in connection with the development of advanced gas turbine engines.
With the introduction of the MCrAlY coatings it was possible to preserve or increase coating and coated component lifetimes in more severe engine environments associated with the advanced engines. As previously mentioned, the MCrAlY coatings are generally deposited on the substrate surface as the MCrAlY alloy usually by vacuum vapor deposition, sputtering or plasma spray techniques. The basic protection is provided by the deposited alloy itself which may be more closely optimized for such protection since it is substantially independent of the substrate itself. There is, of course, a desirable and limited interaction of the coating with the substrate metal but this is in the nature of metallurgical bonding rather than a reaction per se, and the protective elements are derived from the MCrAlY alloy rather than from the substrate.
In later developments it was suggested that additional coating improvements were achievable through the use of multiple coating layers or composite coatings. In the U.S. Patent to Simmons U.S. Pat. No. 3,649,225 of the present assignee, for example, the use of a composite coating comprising a chromium or chromium rich interlayer adjacent a superalloy substrate with an MCrAlY layer thereover is described.
Several other developments relating to MCrAlY-type coatings have even more recently been published in the patent literature. In U.s. Pat. No. 3,849,865 a substrate to be protected is first clad with a metallic foil, such as NiCrAlSi or FeCrAlY and then that foil is covered with an aluminide layer. U.S. Pat. Nos. 3,873,347 and 3,874,901 both appear to describe somewhat similar systems, referring to coating techniques where a superalloy body is first coated with an MCrAlY-type layer which is then aluminized to provide an overlayer of aluminum or an aluminide.